Problem: Stephanie is 3 times as old as Daniel. Eight years ago, Stephanie was 7 times as old as Daniel. How old is Daniel now?
We can use the given information to write down two equations that describe the ages of Stephanie and Daniel. Let Stephanie's current age be $s$ and Daniel's current age be $d$ The information in the first sentence can be expressed in the following equation: $s = 3d$ Eight years ago, Stephanie was $s - 8$ years old, and Daniel was $d - 8$ years old. The information in the second sentence can be expressed in the following equation: $s - 8 = 7(d - 8)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $d$ , it might be easiest to use our first equation for $s$ and substitute it into our second equation. Our first equation is: $s = 3d$ . Substituting this into our second equation, we get: $3d$ $-$ $8 = 7(d - 8)$ which combines the information about $d$ from both of our original equations. Simplifying the right side of this equation, we get: $3 d - 8 = 7 d - 56$ Solving for $d$ , we get: $4 d = 48.$ $d = 12$.